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Sibu New Member Offline Posts: 3	cap and resistance as a function of frequency Apr 21^st, 2010, 8:06pm Hi All, Could anyone teach me how to simulate ac response in LTspice/Spice in which the circuit elements �capacitor and resistor are the function of frequency? I have also included a schematic for your reference. The spice does not like the way I specified the "Freq" or "frequency". Thank you Sibu,
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Geoffrey_Coram

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Re: cap and resistance as a function of frequency
Reply #1 - Apr 22^nd, 2010, 11:55am

I think you don't really understand what a small-signal ac analysis is. The short story is, the dc operating point is computed and then the circuit is linearized about that operating point.

If the resistors depend on frequency, then the dc operating point depends on frequency, and the whole basis for an ac analysis goes out the window.

You could read Ken's book ("Designer's Guide to Spice & Spectre"). You might want to spend some time thinking about what you really want to know.

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love_analog

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Re: cap and resistance as a function of frequency
Reply #2 - Apr 26^th, 2010, 11:10am

Maybe what you are looking for is a broadband model instead of a narrow band one.

We do that sometimes in inductor models in LC VCO.

The way to do is to add more components in parallel or serial so you can approximate the desired inductor (in this case) over a wider frequency range.

It is not easy though

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pancho_hideboo

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Re: cap and resistance as a function of frequency
Reply #3 - Apr 26^th, 2010, 11:18am

Sibu wrote on Apr 21^st, 2010, 8:06pm:

Could anyone teach me how to simulate ac response in LTspice/Spice in which the circuit elements capacitor and resistor are the function of frequency?
I have also included a schematic for your reference.
The spice does not like the way I specified the "Freq" or "frequency".

See http://www.designers-guide.org/Forum/YaBB.pl?num=1227194040/1#1

But I don't know whether "freq" is available or not as system variable in LTspice.
Also you have to consider whether it is reasonable or not physically when you use frequecy dependent C and R.

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Sibu

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Re: cap and resistance as a function of frequency
Reply #4 - Apr 26^th, 2010, 3:18pm

Thank Geoff and Love_Analog for the advice.
Could I more ask you some more questions? I am actually try
to simulate the equivalent circuits of MOS capacitors in which
the parallel capacitance and conductance are as a function
of freq (see attached Fig). I heard also that you can extract the
small signal parameter from transient analysis, is that correct.
So far I have not got the right output, maybe you could give me
more advices.

Thanks
Sibu

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accond.JPG

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Sibu New Member Offline Posts: 3	Re: cap and resistance as a function of frequency Reply #5 - Apr 26^th, 2010, 3:23pm Thanks All, Sorry forgot to add the extraction of the ac parameters from transient analysis. So far I have not got the right output, I am not sure the way I set-up. Thanks Sibu
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Ken Kundert Global Moderator Offline Posts: 2384 Silicon Valley	Re: cap and resistance as a function of frequency Reply #6 - Apr 26^th, 2010, 4:21pm You might want to take a look at Modeling dielectric absorption in capacitors (http://www.designers-guide.org/Modeling/da.pdf). It talks about how to properly model realistic frequency dependence. -Ken
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